Method for performing similar-information search while keeping content confidential by encryption

ABSTRACT

A similar-information search method includes obtaining an instruction indicating which of an encrypted search and an unencrypted search a data computation device is to perform; generating an encrypted feature quantity and transmitting the encrypted feature quantity to the data computation device; generating an encrypted similarity value and transmitting the encrypted similarity value; and receiving encrypted reference data from the data computation device and decrypting the encrypted reference data. A homomorphic encryption scheme that makes decryption by the data computation device unsuccessful is used when the encrypted search is performed, whereas an encryption scheme that makes decryption by the data computation device successful is used when the unencrypted search is performed.

BACKGROUND 1. Technical Field

The present disclosure relates to a method, a device, and a system for performing a similar-information search while keeping content confidential by encryption.

2. Description of the Related Art

Recently, systems have been available in which a user measures, for example, blood pressure at home and a server provides advice about diet or the like to the user on the basis of an analysis of blood pressure. Advice that is more valuable and convincing for the user can be provided if not only information about the user previously obtained but also similar cases are referred to during the analysis before the advice is provided. From the viewpoint of privacy protection, the information about the user and information about other cases that are referred to are desirably kept confidential by encryption when the server searches for similar cases. Japanese Unexamined Patent Application Publication Nos. 2014-126865 and 2014-126866 and Japanese Patent No. 5542474 disclose techniques for performing, using homomorphic encryption, a similar-information search while maintaining encryption.

SUMMARY

Further improvements have been needed for a similar-information search in the related art.

One non-limiting and exemplary embodiment provides a similar-information search method, a terminal device, and a similar-information search system capable of flexibly selecting the way how a search is performed in accordance with a user's desire.

In one general aspect, the techniques disclosed here feature a method, including a terminal device obtaining an instruction indicating which of an encrypted search, in which a server device performs the search using the information provided by the terminal device in an encrypted state, and an unencrypted search, in which the server device performs the search using the information provided by the terminal device in an unencrypted state, is to be performed; the terminal device generating a first encrypted feature quantity by encrypting, using an encryption scheme dependent on the obtained instruction, a first feature quantity indicating a feature of obtained data, the encryption scheme dependent on the obtained instruction being a first encryption scheme that is homomorphic and makes decryption by the server device unsuccessful in a case where the instruction is an instruction to perform an encrypted search and being a second encryption scheme that makes decryption by the server device successful in a case where the instruction is an instruction to perform an unencrypted search; the terminal device transmitting the generated first encrypted feature quantity to the server device; the terminal device generating an encrypted similarity value by encrypting, using the encryption scheme dependent on the instruction, a similarity value indicating a degree of similarity; the terminal device transmitting the generated encrypted similarity value to the server device; the terminal device transmitting the obtained instruction to the server device; the terminal device receiving encrypted reference data from the server device, the encrypted reference data being information obtained by encrypting reference data that is obtained as a result of the server device searching for similar information in accordance with the instruction; and the terminal device decrypting the received encrypted reference data.

It should be noted that general or specific embodiments may be implemented as a system, a device, an integrated circuit, a computer program, or a storage medium such as a computer-readable compact disc-read only memory (CD-ROM), or any selective combination thereof.

According to the disclosed embodiments, the way how a search is performed can be flexibly selected in accordance with a user's desire.

Additional benefits and advantages of the disclosed embodiments will become apparent from content of the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram illustrating an overview of a service provided by a similar-information search system in accordance with embodiments;

FIG. 1B is a block diagram illustrating an example in which a device manufacturer serves as a data center operating company in accordance with the embodiments;

FIG. 1C is a block diagram illustrating an example in which both of or one of the device manufacturer and a management company serve as the data center operating company in accordance with the embodiments;

FIG. 2 is a block diagram illustrating an overall configuration of the similar-information search system in accordance with a first embodiment;

FIG. 3 is a block diagram illustrating a configuration of a user terminal device in accordance with the first embodiment;

FIG. 4 is a block diagram illustrating a configuration of a data computation device in accordance with the first embodiment;

FIG. 5A is a diagram illustrating an example of a reference feature quantity storage unit of the data computation device in accordance with the first embodiment;

FIG. 5B is a diagram illustrating an example of a reference data storage unit of the data computation device in accordance with the first embodiment;

FIG. 6 is a sequence diagram illustrating an operation of the similar-information search system in accordance with the first embodiment;

FIG. 7A is a diagram illustrating an example of a data format used by the user terminal device and the data computation device in accordance with the first embodiment;

FIG. 7B is a diagram illustrating an example of a data format used by the user terminal device and the data computation device in accordance with the first embodiment;

FIG. 8A is a diagram illustrating an example of a screen displayed on the user terminal device in accordance with the first embodiment;

FIG. 8B is a diagram illustrating an example of a screen displayed on the user terminal device in accordance with the first embodiment;

FIG. 9 is a graph illustrating an example of how a feature quantity is calculated with respect to time, which is displayed by the user terminal device in accordance with the first embodiment;

FIG. 10 is a block diagram illustrating a configuration of a user terminal device in accordance with a second embodiment;

FIG. 11 is a block diagram illustrating a configuration of a data computation device in accordance with the second embodiment;

FIG. 12 is a sequence diagram illustrating an operation of a similar-information search system in accordance with the second embodiment;

FIG. 13 is a block diagram illustrating a configuration of a data computation device in the case where statistical reference data is obtained in accordance with a modification;

FIG. 14 is a block diagram illustrating a configuration of a user terminal device in accordance with a third embodiment;

FIG. 15 is a block diagram illustrating a configuration of a data computation device in accordance with the third embodiment;

FIG. 16 is a sequence diagram illustrating an operation of a similar-information search system in accordance with the third embodiment;

FIG. 17 is a diagram illustrating an example of a data format used by the user terminal device and the data computation device in accordance with the third embodiment;

FIG. 18 is a diagram illustrating an example of a screen displayed on the user terminal device in accordance with the third embodiment;

FIG. 19 is a diagram illustrating an example of a screen displayed on the user terminal device in accordance with the third embodiment;

FIG. 20 is a block diagram illustrating an overview of a service provided by the similar-information search system of a service type 1 (local-data-center-based cloud service);

FIG. 21 is a block diagram illustrating an overview of a service provided by the similar-information search system of a service type 2 (IaaS-based cloud service);

FIG. 22 is a block diagram illustrating an overview of a service provided by the similar-information search system of a service type 3 (PaaS-based cloud service); and

FIG. 23 is a block diagram illustrating an overview of a service provided by the similar-information search system of a service type 4 (SaaS-based cloud service).

DETAILED DESCRIPTION Underlying Knowledge Forming Basis of the Present Disclosure

Examples of a method for performing a similar-information search on encrypted content include a method using homomorphic encryption. In this method, however, a server provides only similar information. It is desired that the server accumulate data, which can be used as reference data, to make use of the data in the future if possible.

Some users consider privacy protection important while considering it acceptable to accumulate information about them in a database to make use of the information for other people or in the future. Some people consider such users be given an incentive. Examples of an incentive include rewarding points, providing more detailed information, and providing more valuable advice.

Given these circumstances, a similar-information search method that flexibly adapts to a user's desire has been considered.

A method including:

a terminal device obtaining an instruction indicating which of an encrypted search, in which a server device performs the search using the information provided by the terminal device in an encrypted state, and an unencrypted search, in which the server device performs the search using the information provided by the terminal device in an unencrypted state, is to be performed;

the terminal device generating a first encrypted feature quantity by encrypting, using an encryption scheme dependent on the obtained instruction, a first feature quantity indicating a feature of obtained data, the encryption scheme dependent on the obtained instruction being a first encryption scheme that is homomorphic and makes decryption by the server device unsuccessful in a case where the instruction is an instruction to perform an encrypted search and being a second encryption scheme that makes decryption by the server device successful in a case where the instruction is an instruction to perform an unencrypted search;

the terminal device transmitting the generated first encrypted feature quantity to the server device;

the terminal device generating an encrypted similarity value by encrypting, using the encryption scheme dependent on the instruction, a similarity value indicating a degree of similarity;

the terminal device transmitting the generated encrypted similarity value to the server device;

the terminal device transmitting the obtained instruction to the server device;

the terminal device receiving encrypted reference data from the server device, the encrypted reference data being information obtained by encrypting reference data that is obtained as a result of the server device searching for similar information in accordance with the instruction; and

the terminal device decrypting the received encrypted reference data.

With this configuration, the way how a search is performed can be flexibly selected in accordance with a user's desire.

In this method, the server device may include a first memory that stores a plurality of reference feature quantities and a second memory that stores a plurality of pieces of reference data each corresponding to one of the plurality of reference feature quantities, and the method may further include

the server device receiving the instruction transmitted from the terminal device;

the server device deciding which of the encrypted search and the unencrypted search is to be performed in accordance with the received instruction;

the server device calculating an encrypted distance indicating a degree of similarity between the first encrypted feature quantity received from the terminal device and a first reference feature quantity that is one of the plurality of reference feature quantities stored in the first memory, in a case where it is decided in the deciding that the encrypted search is to be performed;

the server device determining whether the calculated encrypted distance matches the encrypted similarity value received from the terminal device;

the server device obtaining first reference data from the second memory in a case where it is determined in the determining that the calculated encrypted distance matches the encrypted similarity value, the first reference data being data corresponding to the first reference feature quantity used in the calculating of the encrypted distance;

the server device generating the encrypted reference data by encrypting the obtained first reference data; and

the server device transmitting the generated encrypted reference data to the terminal device.

With this configuration, since the server device calculates an encrypted distance and determines whether the encrypted distance matches an encrypted similarity value, the amount of calculation performed by the terminal device can be reduced. Consequently, this method can reduce a load of calculation processing performed by the terminal device.

In addition, the server device may include a first memory that stores a plurality of reference feature quantities and a second memory that stores a plurality of pieces of reference data each corresponding to one of the plurality of reference feature quantities, and the method may further include

the server device receiving the instruction transmitted from the terminal device;

the server device deciding which of the encrypted search and the unencrypted search is to be performed in accordance with the received instruction;

the server device calculating a distance indicating a degree of similarity between a decrypted feature quantity obtained by decrypting the first encrypted feature quantity received from the terminal device and a first reference feature quantity that is one of the plurality of reference feature quantities, in a case where it is decided in the deciding that the unencrypted search is to be performed;

the server device determining whether the calculated distance matches a decrypted similarity value obtained by decrypting the encrypted similarity value received from the terminal device;

the server device obtaining first reference data from the second memory, the first reference data being data corresponding to the first reference feature quantity used in the calculating of the distance in a case where it is determined in the determining that the calculated distance matches the decrypted similarity value;

the server device generating the encrypted reference data by encrypting the obtained first reference data; and

the server device transmitting the generated encrypted reference data to the terminal device.

With this configuration, since the server device calculates a distance and determines whether the distance matches a similarity value, the amount of calculation performed by the terminal device can be reduced. Consequently, this method can reduce a load of calculation processing performed by the terminal device.

In addition, in this method,

the first encryption scheme may be an encryption scheme in which encryption is performed using a public key corresponding to a secret key of the terminal device, and

the second encryption scheme may be an encryption scheme in which encryption is performed using a public key corresponding to a secret key of the server device.

With this configuration, the public key to be used to encrypt a feature quantity and a similarity value can be switched between a public key of the terminal device and a public key of the server device, depending on which of an encrypted search and an unencrypted search is to be performed. Specifically, when a user wishes to perform a search while keeping information confidential, the setting can be switched so that the server device performs an encrypted search. When a user wishes to perform a more detailed search, the setting can be switched so that the server device performs an unencrypted search.

In addition, this method may further include

the terminal device transmitting registration permission information to the server device, the registration permission information being information indicating whether registration of information about a user of the terminal device in the server device is permitted; and

the server device registering the information about the user transmitted from the terminal device, as at least one of the reference feature quantity and the reference data in a case where the received registration permission information indicates that registration is permitted.

With this configuration, a user can select whether to provide information about them to the server device. Accordingly, the user can obtain a more detailed search result in return for disclosing their information to the server device. Alternatively, the user can accumulate their information in the server device and make use of the information as reference data during a similar-information search performed by another user.

A terminal device including

one or more memories; and

circuitry which, in operation:

-   -   obtains an instruction indicating which of an encrypted search,         in which a server device performs the search using information         provided by the terminal device in an encrypted state, and an         unencrypted search, in which the server device performs the         search using the information provided by the terminal device in         an unencrypted state, is to be performed;     -   generates a first encrypted feature quantity by encrypting,         using an encryption scheme dependent on the obtained         instruction, a first feature quantity indicating a feature of         obtained data, the encryption scheme dependent on the obtained         instruction being a first encryption scheme that is homomorphic         and makes decryption by the server device unsuccessful in a case         where the instruction is an instruction to perform an encrypted         search and being a second encryption scheme that makes         decryption by the server device successful in a case where the         instruction is an instruction to perform an unencrypted search;     -   transmits the generated first encrypted feature quantity to the         server device;     -   generates an encrypted similarity value by encrypting, using the         encryption scheme dependent on the instruction, a similarity         value indicating a degree of similarity;     -   transmits the generated encrypted similarity value to the server         device;     -   transmits the obtained instruction to the server device;     -   receives encrypted reference data from the server device, the         encrypted reference data being information obtained by         encrypting reference data that is obtained as a result of the         server device searching for similar information in accordance         with the instruction; and     -   decrypts the received encrypted reference data.

A system according to an aspect of the present disclosure is a similar-information search system including

a terminal device that provides information; and

a server device that searches for similar information, based on the provided information,

the terminal device including

one or more memories; and

circuitry which, in operation:

-   -   obtains an instruction indicating which of an encrypted search,         in which the server device performs the search using the         information provided by the terminal device in an encrypted         state, and an unencrypted search, in which the server device         performs the search using the information provided by the         terminal device in an unencrypted state, is to be performed;     -   generates a first encrypted feature quantity by encrypting,         using an encryption scheme dependent on the obtained         instruction, a first feature quantity indicating a feature of         obtained data, the encryption scheme dependent on the obtained         instruction being a first encryption scheme that is homomorphic         and makes decryption by the server device unsuccessful in a case         where the instruction is an instruction to perform an encrypted         search and being a second encryption scheme that makes         decryption by the server device successful in a case where the         instruction is an instruction to perform an unencrypted search;

transmits the generated first encrypted feature quantity to the server device;

generates an encrypted similarity value by encrypting, using the encryption scheme dependent on the instruction, a similarity value indicating a degree of similarity;

transmits the generated encrypted similarity value to the server device;

transmits the obtained instruction to the server device;

receives encrypted reference data from the server device, the encrypted reference data being information obtained by encrypting reference data that is obtained as a result of the server device searching for similar information in accordance with the instruction; and

decrypts the received encrypted reference data.

With the terminal device and the system, the way how a search is performed can be flexibly selected in accordance with a user's desire.

Embodiments described hereinafter provides a specific example of the present disclosure. The values, shapes, components, steps, orders of the steps, etc., given in the following embodiments are illustrative, and are not intended to limit the present disclosure. In addition, among the components in the following embodiments, a component not recited in any of the independent claims indicating the most generic concept of the present disclosure is described as an optional component. In addition, every embodiment may be combined with every other embodiment.

Overview of Provided Service

An overview of a service provided by a similar-information search system according to embodiments will be described first with reference to FIGS. 1A, 1B, and 1C.

FIG. 1A is a block diagram illustrating an overview of a service provided by a similar-information search system in accordance with the embodiments. FIG. 1B is a block diagram illustrating an example in which a device manufacturer serves as a data center operating company in accordance with the embodiments. FIG. 10 is a block diagram illustrating an example in which both of or one of the device manufacturer and a management company serve as the data center operating company in accordance with the embodiments.

The similar-information search system includes a group 1000, a data center operating company 1110, and a service provider 1120.

The group 1000 may be, for example, a company, an organization, or a home and may be a group of any given scale. The group 1000 includes a plurality of devices 1101 including a first device and a second device, and a home gateway 1102. Examples of the plurality of devices 1101 include devices connectable to the Internet (e.g., a smartphone, a personal computer (PC), and a television) and devices not connectable to the Internet on their own (e.g., a light, a washing machine, and a refrigerator). The plurality of devices 1101 may include devices that are not connectable to the Internet on their own but are connectable to the Internet via the home gateway 1102. Users 1010 use the plurality of devices 1101 in the group 1000.

The data center operating company 1110 includes a cloud server 1111. The cloud server 1111 is a virtual server that operates in cooperation with various devices via the Internet. Typically, the cloud server 1111 manages big data that is difficult to handle with an ordinary database management tool or the like. The data center operating company 1110 performs management of data, management of the cloud server 1111, and operation of the data center where such management is performed. Details of services provided by the data center operating company 1110 will be described later.

The data center operating company 1110 is not necessary a company that performs only management of data or management of the cloud server 1111. For example, in the case where a device manufacturer that develops or manufactures one of the plurality of devices 1101 also performs management of data or management of the cloud server 1111 as illustrated in FIG. 1B, the device manufacturer serves as the data center operating company 1110. In addition, the data center operating company 1110 is not necessarily run by one company. For example, in the case where a device manufacturer and a management company perform management of the data or management of the cloud server 1111 together or in a shared manner as illustrated in FIG. 1C, both of or one of the device manufacturer and the management company serve as the data center operating company 1110.

The service provider 1120 includes a server 1121. Herein, the server 1121 may be a server of any given scale. Examples of the server 1121 include a memory included in a PC. In addition, the service provider 1120 does not necessarily include the server 1121.

Note that the home gateway 1102 is not mandatory to the similar-information search system. For example, the home gateway 1102 is not needed in the case where the cloud server 1111 performs management of all data. In addition, there may be a case where devices that are not connectable to the Internet on their own do not exist as in the case where every device at home is connected to the Internet.

Now, the flow of information in the similar-information search system will be described.

First, the first device or the second device in the group 1000 transmits log information to the cloud server 1111 of the data center operating company 1110. The cloud server 1111 accumulates the log information of the first device or the second device (arrow 1131 in FIG. 1A). Note that log information is, for example, information indicating the operating state or the operation date/time of the plurality of devices 1101. Examples of the log information include a history of viewing a television, information about scheduled recording in a recorder, an operation date/time of a washing machine, a weight of laundry, an opening/closing date/time of a refrigerator, and the number of times the refrigerator is opened and closed; however, the log information is not limited to these pieces of information and may include various kinds information that can be obtained from various devices. The log information may be provided directly to the cloud server 1111 from the plurality of devices 1101 via the Internet, or may be temporarily accumulated in the home gateway 1102 from the plurality of devices 1101 and then provided to the cloud server 1111 from the home gateway 1102.

Then, the cloud server 1111 of the data center operating company 1110 provides the accumulated log information to the service provider 1120 in a certain unit. The certain unit may be a unit in which the data center operating company 1110 can organize and provide the accumulated information to the service provider 1120 or a unit requested by the service provider 1120. The log information need not necessarily be provided in the certain unit, and an amount of provided information may change depending on the circumstances. The log information is stored in the server 1121 of the service provider 1120 if necessary (arrow 1132 in FIG. 1A).

Then, the service provider 1120 organizes the log information into information suitable for the service provided to the user, and provides the information to the user. The user provided with the information may be the user 1010 who uses the plurality of devices 1101 or may be an outside user (hereinafter, simply referred to as a user) 1020. As for the method for providing the information to the users 1010 and 1020, the information may be provided directly to the users 1010 and 1020 from the service provider 1120 (dashed-line arrows 1133 and 1134 in FIG. 1A), for example. As for the method for providing the service to the user 1010, the information may be provided to the user 1010 via the cloud server 1111 of the data center operating company 1110 (arrows 1135 and 1136 in FIG. 1A), for example. In addition, the cloud server 1111 of the data center operating company 1110 may organize the log information into information suitable for the service provided to the users 1010 and 1020 and then provide the resulting information to the service provider 1120.

Note that the user 1010 and the user 1020 may be the same user or different users.

First Embodiment

A similar-information search system related to the present disclosure will now be described as a first embodiment of the present disclosure, with reference to the drawings.

1.1 Overall Configuration of Similar-Information Search System

FIG. 2 is a block diagram illustrating an overall configuration of a similar-information search system in accordance with a first embodiment.

As illustrated in FIG. 2, a similar-information search system 10 includes a user terminal device 100, which is an example of a terminal device, and a data computation device 200, which is an example of a server device. The user terminal device 100 requests the data computation device 200 to perform a search. The data computation device 200 searches for similar information on the basis of information provided by the user terminal device 100 and provides the retrieved similar information to the user terminal device 100.

The similar-information search system 10 may include a plurality of user terminal devices 100 and a plurality of data computation devices 200. For example, the plurality of user terminal devices 100 may be connected to one data computation device 200, or the plurality of data computation devices 200 may be connected to one user terminal device 100. In addition, the user terminal device 100 and the data computation device 200 may be connected in any given manner, such as via a wired Internet line, via wireless communication, or via dedicated communication. Note that communication does not necessarily need to be performed in real time in this similar-information search system 10. For example, the user terminal device 100 may collect a certain amount of information by sensing or a certain number of similar-information search requests and then collectively transmit the information or the requests to the data computation device 200.

1.2 User Terminal Device

FIG. 3 is a block diagram illustrating a configuration of the user terminal device 100 in accordance with the first embodiment.

The user performs an encrypted search in which a search is performed using encrypted information. That is, the user terminal device 100 requests the data computation device 200 that searches for similar information to perform a search. As illustrated in FIG. 3, the user terminal device 100 includes a sensing unit 101, a feature quantity calculation unit 102, a feature quantity encryption unit 103, a similarity value setting unit 104, a similarity value encryption unit 105, a reference data utilization unit 106, a decryption unit 107, and a key storage unit 108.

The feature quantity encryption unit 103, the similarity value encryption unit 105, and the decryption unit 107 are connected to corresponding components (indicated by A, B, and C, respectively) of the data computation device 200 illustrated in FIG. 4. The user terminal device 100 performs sensing to obtain information about a user, such as blood pressure information, facial color information, and computed tomography (CT) scan information, and extracts a feature quantity indicating a feature of the obtained data. The user terminal device 100 then requests the data computation device 200 to provide reference data of a symptom similar to this feature quantity and utilizes the obtained reference data in the service provided to the user. The user terminal device 100 obtains a certain type of agreement from the user before it performs sensing to obtain information about the user. Note that a procedure of obtaining the agreement is not within the scope of the present disclosure.

1.2.1 Sensing Unit

The sensing unit 101 performs sensing on a user. Examples of data obtained by sensing include data of vital signs such as blood pressure and temperature of the user, image data such as data of a user's face image, an ultrasound image, and a CT scan image, and answering voice data. In addition, the data obtained by sensing may be user's location information (global positioning system (GPS) information), shopping log information, or the like.

1.2.2 Feature Quantity Calculation Unit

The feature quantity calculation unit 102 calculates a feature quantity from the data obtained by the sensing unit 101. The calculated feature quantity may be, for example, data such as a main component of a characteristic parameter in image information of a face or the like or a position, area, or width of a certain area. In addition, the feature quantity may be, for example, a coefficient of a polynomial obtained by converting a tendency of log information with respect to the time axis. Further, the feature quantity may be a numerical value or a vector value including a list of numerical values.

1.2.3 Feature Quantity Encryption Unit

The feature quantity encryption unit 103 encrypts the feature quantity calculated by the feature quantity calculation unit 102 and denoting a feature of the data to generate an encrypted feature quantity. The feature quantity encryption unit 103 also transmits the generated encrypted feature quantity to the data computation device 200. It is assumed that the encryption scheme used is homomorphic encryption, which allows the data computation device 200 to perform computation for a similar-information search by using the encrypted feature quantity. When the feature quantity is kept confidential from the data computation device 200, a public key of the user terminal device 100, which is stored in the key storage unit 108 (described later), is used.

1.2.4 Similarity Value Setting Unit

The similarity value setting unit 104 sets a similarity value that indicates a degree of similarity used in a search. The term “similarity value” used herein refers to a value indicating the degree of similarity between to the feature quantity and information to be searched for by the data computation device 200. For example, the data computation device 200 calculates Euclidean distance between the feature quantity and a reference feature quantity. In the case where information for which the Euclidean distance is smaller than a certain value D is searched for, the certain value D is set as the similarity value. Alternatively, all the values 0, 1, . . . , D−1 smaller than the certain value D may be set as the similarity values. Alternatively, the similarity value may be set for each vector component value of the feature quantity.

1.2.5 Similarity Value Encryption Unit

The similarity value encryption unit 105 encrypts the similarity value, which indicates the degree of similarity, to generate an encrypted similarity value. The similarity value encryption unit 105 also transmits the generated encrypted similarity value to the data computation device 200. The encrypted similarity value is used by the data computation device 200 to determine whether a reference feature quantity is similar to the encrypted feature quantity in an encrypted state and to extract a reference ID that satisfies a similarity condition.

1.2.6 Decryption Unit

The decryption unit 107 receives, from the data computation device 200, encrypted reference data, which is information obtained by encrypting reference data (similar-information search result) that is obtained based on the encrypted feature quantity and the encrypted similarity value through the similar-information search requested by the user terminal device 100. The decryption unit 107 then decrypts the received encrypted reference data.

1.2.7 Reference Data Utilization Unit

The reference data utilization unit 106 utilizes reference data obtained by the decryption unit 107 through decryption. For example, the reference data utilization unit 106 displays a graph or a statistical value of the reference data on the user terminal device 100 or provides the user with advice about how to improve the lifestyle or about recommended diet by using such information.

1.2.8 Key Storage Unit

The key storage unit 108 stores keys used by the feature quantity encryption unit 103, the similarity value encryption unit 105, and the decryption unit 107. The encryption schemes and the keys used by the feature quantity encryption unit 103, the similarity value encryption unit 105, and the decryption unit 107 may be the same or may be different. For example, the key used by the feature quantity encryption unit 103 and the similarity value encryption unit 105 may be the public key of the user terminal device 100 used in a homomorphic encryption scheme. Note that the corresponding secret key is also stored in the key storage unit 108. The key used by the decryption unit 107 may be the secret key of the user terminal device 100 used in a public key encryption scheme that is not the homomorphic encryption scheme. Note that the corresponding public key is shared with the data computation device 200. Alternatively, the key used by the decryption unit 107 may be the secret key of the user terminal device 100 used in the homomorphic encryption scheme just like the key used by the feature quantity encryption unit 103.

1.3 Data Computation Device

FIG. 4 is a block diagram illustrating a configuration of the data computation device 200 in accordance with the first embodiment.

The data computation device 200 searches, in response to a search request from a user, for similar information on the basis of information provided by the user terminal device 100 and provides the retrieved information to the user terminal device 100. As illustrated in FIG. 4, the data computation device 200 includes a reference feature quantity storage unit 201, an encrypted distance calculation unit 202, a reference similarity value extraction unit 203, an encrypted reference data extraction unit 204, and a reference data storage unit 205. The encrypted distance calculation unit 202, the reference similarity value extraction unit 203, and the encrypted reference data extraction unit 204 are connected to corresponding components (indicated by A, B, and C, respectively) of the user terminal device 100 illustrated in FIG. 3.

1.3.1 Reference Feature Quantity Storage Unit

FIG. 5A is a diagram illustrating an example of the reference feature quantity storage unit 201 of the data computation device 200 in the first embodiment. FIG. 5A illustrates an example of reference feature quantities stored in the reference feature quantity storage unit 201.

As illustrated in FIG. 5A, the reference feature quantity storage unit 201 stores reference feature quantities Yi to be searched when the data computation device 200 performs a similar-information search. Note that an identifier ID corresponds to “i”, and each reference feature quantity is denoted by Yi. The identifier ID and the reference feature quantity are stored in the reference feature quantity storage unit 201 in pairs. In FIG. 5A, the reference feature quantities Y1, Y2, . . . respectively corresponding to the identifiers IDs 1, 2, . . . are stored. It is assumed in this example that each reference feature quantity is denoted as a vector value. Note that the reference feature quantities may be encrypted before they are stored.

1.3.2 Encrypted Distance Calculation Unit

The encrypted distance calculation unit 202 receives, from the user terminal device 100, an encrypted feature quantity, which is obtained by encryption using the homomorphic encryption scheme. The encrypted distance calculation unit 202 calculates an encrypted distance, which indicates a degree of similarity between the received encrypted feature quantity and at least one of the plurality of reference feature quantities, while keeping the received feature quantity and the reference feature quantities encrypted. That is, the encrypted distance calculation unit 202 calculates a distance between the feature quantity generated by the user terminal device 100 and a reference feature quantity stored in the data computation device 200. Specifically, the encrypted distance calculation unit 202 encrypts a reference feature quantity received from the reference feature quantity storage unit 201. The encrypted distance calculation unit 202 then calculates a distance between the encrypted feature quantity received from the user terminal device 100 and the encrypted reference feature quantity, while keeping these feature quantities encrypted.

The encrypted distance calculation unit 202 then outputs the encrypted distance, which is a distance that has been encrypted, to the reference similarity value extraction unit 203. The encrypted distance may be, for example, an absolute value of a difference between these feature quantities if the feature quantities are numerical values or the sum of absolute values of differences between the corresponding vector components if the feature quantities are vectors including numerical values as their components. Alternatively, the encrypted distance may be a cosine similarity, which is a sum of the products of the corresponding vector components.

Specifically, suppose that the feature quantity and the reference feature quantity are vectors having numerical values as their components and are denoted as (x1, x2, x3, . . . , xn) and (y11, y12, y13, . . . , y1 n), respectively. In such a case, the distance obtained by adding absolute values of differences between the corresponding vector components is denoted as Σ|xi−y1 i|, the cosine similarity is denoted as Σxi×Σy1 i, and the Euclidean distance is denoted as Σ(xi−y1 i)̂2. Since the encrypted distance calculation unit 202 performs this calculation with the feature quantity and the reference feature quantity kept encrypted, the homomorphic encryption scheme is used. During computation using the homomorphic encryption scheme, the calculation may be performed for each component of the vector value. Alternatively, to speed up the calculation, the distance may be calculated using the homomorphic encryption scheme by performing computations on polynomials in each of which a vector value of the corresponding feature quantity is mapped as a coefficient.

1.3.3 Reference Similarity Value Extraction Unit

The reference similarity value extraction unit 203 receives the encrypted similarity value from the similarity value encryption unit 105. The encrypted similarity value is information obtained by encrypting a similarity value, which indicates the degree of similarity. Note that this encrypted information is preferably obtained by encryption using the homomorphic encryption scheme.

The reference similarity value extraction unit 203 determines whether the distance matches the similarity value by using the encrypted similarity value received from the similarity value encryption unit 105 and the encrypted distance received from the encrypted distance calculation unit 202. Note that the encrypted distance matching the encrypted similarity value is synonymous with the distance matching the similarity value. If the reference similarity value extraction unit 203 determines that the distance matches the similarity value, the reference similarity value extraction unit 203 extracts the identifier ID of the corresponding reference feature quantity. If the reference similarity value extraction unit 203 determines that the encrypted distance does not match the encrypted similarity value, the reference similarity value extraction unit 203 does not extract the identifier ID of the corresponding reference feature quantity.

1.3.4 Reference Data Storage Unit

FIG. 5B is a diagram illustrating an example of the reference data storage unit 205 of the data computation device 200 in the first embodiment. FIG. 5B illustrates an example of the identifiers IDs and pieces of reference data stored in the reference data storage unit 205.

As illustrated in FIG. 5B, the reference data storage unit 205 stores pieces of reference data to be searched by the data computation device 200 to perform a similar-information search. The identifier ID and the reference data are stored in the reference data storage unit 205 in pairs. The pieces of reference data Z1, Z2, . . . , Zi respectively corresponding to the identifiers IDs 1, 2, . . . , i are stored in the reference data storage unit 205. Note that the identifier ID corresponds to “i”, and the reference data is denoted by Zi. Examples of the reference data include a history regarding diseases and treatments (reference data in FIG. 5B) corresponding to the feature quantity (reference feature quantity in FIG. 5A) that indicates the tendency of measured blood pressure values and a history regarding purchased products (reference data in FIG. 5B) corresponding to the feature quantity (reference feature quantity in FIG. 5A) that indicates the user location information. In the example in FIG. 5B, histories regarding diseases and treatments, such as a user assigned the identifier ID of 1 having been hospitalized in October, 2013 for a cerebral infarction and a user assigned the identifier of 2 having been started a treatment for diabetes in February, 1999, are accumulated. The reference data may be encrypted before it is stored.

1.3.5 Encrypted Reference Data Extraction Unit

The encrypted reference data extraction unit 204 generates encrypted reference data if the reference similarity value extraction unit 203 determines that the distance matches the similarity value. The encrypted reference data is information obtained by encrypting reference data, which is data corresponding to the reference feature quantity used in calculation of the encrypted distance. The encrypted reference data extraction unit 204 also transmits the generated encrypted reference data to the decryption unit 107 of the user terminal device 100. Specifically, the encrypted reference data extraction unit 204 extracts, from the reference data storage unit 205, reference data corresponding to the identifier ID extracted by the reference similarity value extraction unit 203 and transmits the encrypted reference data to the decryption unit 107 of the user terminal device 100. Before transmitting the reference data, the encrypted reference data extraction unit 204 encrypts the reference data to protect the data on a communication channel. Before performing encryption, the encrypted reference data extraction unit 204 may make the user corresponding to the reference data anonymous to make the user unidentifiable based on the reference data. In addition, the data computation device 200 may derive statistical information, such as an average or a frequency, from a plurality of pieces of extracted reference data and transmit the statistical information to the decryption unit 107 of the user terminal device 100 after encrypting the statistical information.

1.4 Operation of Similarity-Information Search System 1.4.1 Overview of Operation

An operation of the similar-information search system 10 will be described with reference to FIG. 6.

FIG. 6 is a sequence diagram illustrating an operation of the similar-information search system 10 in accordance with the first embodiment.

As illustrated in FIG. 6, in step S101, the sensing unit 101 of the user terminal device 100 performs sensing on a user and obtains data about the user.

In step S102, the feature quantity calculation unit 102 calculates a feature quantity X from the data obtained by the sensing performed by the sensing unit 101. For example, the feature quantity X may be an m-dimensional vector whose components have following numerical values.

X=(x1, x2, . . . , xm)

In step S103, the similarity value setting unit 104 sets a similarity value that serves as a criterion during a search. For example, in the case where data for which the distance is less than D is searched for, distances 0, 1, 2, . . . , D−1 are set as the similarity values.

In step S104, the feature quantity encryption unit 103 encrypts the feature quantity X using the homomorphic encryption scheme to generate an encrypted feature quantity. In addition, the similarity value encryption unit 105 encrypts the similarity value using the homomorphic encryption scheme to generate an encrypted similarity value. The homomorphic encryption scheme is an encryption scheme that makes decryption by the data computation device 200 unsuccessful. The key used during encryption is the public key of the user terminal device 100.

Let Enc denote an encryption function. Then, the encrypted feature quantity is denoted as

Ex=(Enc(x1), Enc(x2), . . . , Enc(xm)), and

the encrypted similarity values can be denoted as

Enc(0), Enc(1), . . . , Enc(D−1).

In step S105, the user terminal device 100 requests the data computation device 200 to provide similar reference data that satisfies a condition, by providing the encrypted feature quantity and the encrypted similarity value to the data computation device 200. Specifically, the encrypted feature quantity is transmitted to the encrypted distance calculation unit 202, and the encrypted similarity value is transmitted to the reference similarity value extraction unit 203.

Specifically, the encrypted distance calculation unit 202 receives, from the feature quantity encryption unit 103, the encrypted feature quantity that is a feature quantity encrypted using the homomorphic encryption scheme. The reference similarity value extraction unit 203 receives, from the similarity value encryption unit 105, the encrypted similarity value that is information obtained by encrypting the similarity value indicating the degree of similarity. Note that the encrypted similarity value that is the encrypted information is preferably obtained by encryption using the homomorphic encryption scheme.

In addition, a plurality of encrypted similarity values may be received each of which is information obtained by encrypting a corresponding one of a plurality of similarity values indicating a plurality of degrees of similarity.

In step S106, the data computation device 200 selects the reference feature quantity Yi=(yi1, yi2, . . . , yim) assigned an i-th identifier ID from the reference feature quantity storage unit 201.

In step S107, the encrypted distance calculation unit 202 calculates a distance indicating a degree of similarity between at least one encrypted reference feature quantity, obtained by encrypting at least one of the plurality of reference feature quantities, and the encrypted feature quantity received by the data computation device 200, with the feature quantities kept encrypted. The encrypted reference feature quantity can be denoted as

Ey=(Enc(yi1), Enc(yi2), . . . , Enc(yim)).

Note that the at least one encrypted reference feature quantity, obtained by encrypting at least one of the plurality of reference feature quantities, is preferably obtained by encryption using the homomorphic encryption scheme.

The encrypted distance calculation unit 202 calculates, by determining differences between the corresponding vector components of the feature quantity X and the reference feature quantity Yi and adding the differences, by using the property of the homomorphic encryption scheme, the encrypted distance EDi denoted as follows:

EDi=Enc(|x1−yi1|+|x2−yi2|+ . . . +|xm−yim|).

In the above, the sum of absolute values of differences between the corresponding vector components is used as the encrypted distance. Note that the data computation device 200 may perform step S107 for each of the plurality of encrypted similarity values.

In step S108, the reference similarity value extraction unit 203 checks whether a condition is satisfied by using the encrypted distance and the encrypted similarity value. Specifically, in step S108, it is determined whether the calculated encrypted distance matches the encrypted similarity value received by the data computation device 200. Note that the data computation device 200 may perform step S108 for each of the plurality of encrypted similarity values.

For example, it is checked whether the encrypted distance EDi matches any of the encrypted similarity values Enc(0), Enc(1), . . . , Enc(D−1). If the encrypted distance EDi matches any of the encrypted similarity values, it indicates that content of the encryption function Enc of the encrypted distance EDi, that is, the distance between the feature quantity and the reference feature quantity (|x1−yi1|+|x2−yi2|+ . . . +|xm−yim|) is one of the distances 0, 1, . . . , D−1 that serve as the similarity values.

If the encrypted distance EDi matches none of the encrypted similarity values, that is, the condition is not satisfied, the next identifier ID is selected. The process then returns to step S106.

If the encrypted distance EDi matches any of the encrypted similarity values, that is, the condition is satisfied, the reference similarity value extraction unit 203 records the current identifier ID in step S109 because the distance between these feature quantities is smaller than the distance D. Steps S106 to S109 are repeatedly performed for each identifier ID in the reference feature quantity storage unit 201.

If it is determined in S108 that the encrypted distance matches any of the encrypted similarity values, the encrypted reference data extraction unit 204 generates encrypted reference data, which is information obtained by encrypting reference data that is data corresponding to the reference feature quantity used in calculation of the encrypted distance, in step S110. Specifically, in step S110, the encrypted reference data extraction unit 204 extracts, from the reference data storage unit 205, the reference data corresponding to the identifier ID extracted in step S109 and encrypts the reference data by using the public key of the user terminal device 100.

In step S111, the encrypted reference data extraction unit 204 transmits the generated encrypted reference data to the user terminal device 100. Note that the encrypted reference data extraction unit 204 may perform step S111 for each of the plurality of encrypted similarity values.

In step S112, the decryption unit 107 of the user terminal device 100 receives the encrypted reference data from the encrypted reference data extraction unit 204. The decryption unit 107 then decrypts the encrypted reference data received from the data computation device 200, and the reference data utilization unit 106 utilizes the resulting reference data.

1.4.2 Feature Quantity Calculation Example

In the case where image data, such as image data of a face or ultrasound information, is obtained by the sensing unit 101, the feature quantity calculated in step S102 may be, for example, a main component of a characteristic parameter in such image information or a position, area, or width of a certain area. In the case where blood pressure information is obtained, a tendency with respect to the time axis is converted into, for example, a polynomial from a history (not illustrated in FIG. 3) of the measured blood pressure information that has been obtained and stored separately in the user terminal device 100, and a coefficient of the polynomial may be used as the feature quantity. FIG. 9 illustrates an example of the polynomial denoting the tendency with respect to the time axis.

FIG. 9 is a graph illustrating an example of how the feature quantity is calculated with respect to time, which is displayed by the user terminal device 100 in accordance with the first embodiment. In FIG. 9, the horizontal axis illustrates a time period from a past time point (e.g. one year ago) to the present time point. In addition, in FIG. 9, an approximation curve is derived from measured values (denoted by x), and the coefficient of the approximation curve is used as the feature quantity.

1.4.3 Encrypted Distance Calculation Example

In the above, the sum of the differences between the corresponding vector component values of the feature quantity determined by the user terminal device 100 and the reference feature quantity stored in the data computation device 200 is determined as the encrypted distance in step S107, with the feature quantities kept encrypted. The encrypted distance is Euclidean distance between the reference feature quantity and the encrypted feature quantity. Alternatively, for example, a method for calculating a cosine similarity may be used to calculate the encrypted distance. In this case, fully homomorphic encryption that allows both addition and multiplication to be performed with the feature quantities kept encrypted or limited homomorphic encryption for which the number of times of multiplication is limited (somewhat homomorphic encryption) may be used.

1.4.4 Data Exchanged between User Terminal Device and Data Computation Device

FIG. 7A is a diagram illustrating an example of a data format used by the user terminal device 100 and the data computation device 200 in accordance with the first embodiment. FIG. 7B is a diagram illustrating an example of a data format used by the user terminal device 100 and the data computation device 200 in accordance with the first embodiment. FIGS. 7A and 7B illustrate examples of the format of data exchanged between the user terminal device 100 and the data computation device 200 in steps S105 and S111, respectively. The data format includes fields for an identifier ID of a source, an identifier ID of a destination, a command, and data. Note that when commands are exchanged between the plurality of user terminal devices 100 and the plurality of data computation devices 200 in parallel, an identifier may be added to indicate a correspondence between steps S105 and S111.

1.4.5 Display Example

FIG. 8A is a diagram illustrating an example of a screen displayed on the user terminal device 100 in accordance with the first embodiment. FIG. 8B is a diagram illustrating an example of a screen displayed on the user terminal device 100 in accordance with the first embodiment.

FIG. 8A illustrates an example of a screen displayed on the user terminal device 100 in step S101 in FIG. 6.

In this example, when the user clicks (selects) “Start”, the sensing unit 101 starts measuring blood pressure. Then, the user selects whether to search for similar cases on the basis of their recent blood pressure tendency through “Yes” or “Cancel”. If the user clicks “Yes” to show an agreement, the user terminal device 100 calculates the feature quantity from the tendency by using the measurement value and recent log information. If the user clicks “Cancel” to show a disagreement, the user terminal device 100 does not transmit information used to search for similar cases to the data computation device 200.

FIG. 8B illustrates an example of a screen displayed when advice about diet and exercise is provided to the user by using the retrieved similar reference data. In this case, the similar reference data denotes diseases which users having a similar blood pressure change tendency have had later.

In this example, the user terminal device 100 displays the tendency of the blood pressure from the past time point to the present time point on the basis of the information about the user and displays the similar reference data. Since the user terminal device 100 provides advice based on the information about user, the advice is more convincing and valuable for the user.

1.5 Advantageous Effects of First Embodiment

In the first embodiment, the data computation device 200 receives the encrypted feature quantity and the encrypted similarity value from the user terminal device 100, and the encrypted reference data extraction unit 204 extracts encrypted reference data. Since the data computation device 200 extracts and transmits, to the user terminal device 100, only the encrypted reference data that satisfies the similarity value smaller than D, an amount of calculation performed by the user terminal device 100 and an amount of data communicated between the data computation device 200 and the user terminal device 100 can be reduced. In addition, the user terminal device 100 can flexibly set the similarity value in accordance with a user's desire.

In addition, since the encrypted feature quantity is obtained by encryption using a homomorphic encryption scheme, the data computation device 200 is unable to decrypt the encrypted feature quantity. For this reason, this similar-information search method or the like can enhance the confidentiality of information searched for by the user during transmission from the user terminal device 100 to the data computation device 200 and in the data computation device 200.

Second Embodiment

A similar-information search system relating to the present disclosure will now be described as a second embodiment of the present disclosure with reference to the drawings. Differences between the second embodiment and the first embodiment are as follows. In the first embodiment, the reference similarity value extraction unit 203 determines whether a condition is satisfied depending on whether the encrypted distance matches the encrypted similarity value in step S108. The assumption of this is the use of a deterministic encryption scheme for which, if original pieces of data are the same, their encrypted pieces of data also match. In contrast, in the second embodiment, a probabilistic encryption scheme is used for which a random number is generated at the time of encryption and data including the random number is encrypted, so that the encrypted pieces of data do not necessarily have the same value even if the original pieces of data are the same. The use of the probabilistic encryption scheme makes an analysis based on a frequency in ciphertexts more difficult and can enhance the security. On the other hand, if a random number is used at the time of encryption in the deterministic encryption scheme of the first embodiment, the determination as to whether the encrypted distance matches the encrypted similarity value can no longer be performed.

In the second embodiment, a description will be given of a method that makes it possible to perform computation for a similar-information search on encrypted information when the probabilistic encryption scheme is used. In the similar-information search system, the user terminal device 100 converts a key for decryption and provides the converted key to the data computation device 200, and the data computation device 200 obtains a converted distance by decryption. Then, in the similar-information search system, a component of the random number is removed by decryption. Thus, it can be determined whether the converted distance matches the converted similarity value. In addition, since the key for decryption is converted by using a one-way function, the data computation device 200 is unable to determine the key. Hereinafter, the differences between the second embodiment and the first embodiment will be mainly described, and each configuration of the second embodiment that is substantially the same as that of the first embodiment is denoted by the same reference sign, and a detailed description about such a configuration is omitted.

2.1 User Terminal Device

FIG. 10 is a block diagram illustrating a configuration of the user terminal device 100 in accordance with the second embodiment.

As illustrated in FIG. 10, the user terminal device 100 includes a similarity value conversion unit 110 in place of the similarity value encryption unit 105 and further includes a key conversion unit 109 in addition to the components of the first embodiment except for the similarity value encryption unit 105. Note that components, in the block diagram of FIG. 10, that are denoted by the same reference signs as those used in FIG. 3 have substantially the same functions as those in FIG. 3, and thus a description thereof is omitted. It is assumed that the feature quantity encryption unit 103, the similarity value conversion unit 110, the key conversion unit 109, and the decryption unit 107 in FIG. 10 are connected to corresponding components (indicated by D, E, F, and G, respectively) of the data computation device 200 illustrated in FIG. 11.

2.1.1 Key Conversion Unit

The key conversion unit 109 calculates a converted key value. The converted key value is information obtained by converting a secret key of the user terminal device 100 by using a one-way function. Examples of the one-way function include exponentiation in which the secret key is used as the exponent. Conversion using a one-way function indicates that it is easy to perform exponentiation using the secret key but it is difficult to do the opposite, that is, to determine the exponent from the result of exponentiation. Determining the exponent from the result of exponentiation ultimately equates to a fact that a logarithmic operation in a discrete logarithmic domain is difficult.

2.1.2 Similarity Value Conversion Unit

The similarity value conversion unit 110 converts the set similarity value by using the same one-way function as that used by the key conversion unit 109.

2.2 Data Computation Device

FIG. 11 is a block diagram illustrating a configuration of the data computation device 200 in accordance with the second embodiment.

As illustrated in FIG. 11, the data computation device 200 includes a reference converted similarity value extraction unit 207 in place of the reference similarity value extraction unit 203 of the first embodiment and further includes a converted distance decryption unit 206 in addition to the components of the first embodiment except for the reference similarity value extraction unit 203. Note that components, in the block diagram of FIG. 11, that are denoted by the same reference signs as those used in FIG. 4 have substantially the same functions as those in FIG. 4, and thus a description thereof is omitted. It is assumed that D, E, F, and G in FIG. 11 are connected to the corresponding components illustrated in FIG. 10.

2.2.1 Converted Distance Decryption Unit

The converted distance decryption unit 206 determines a converted distance by decrypting an encrypted distance by using the converted key value, which is information obtained as a result of the key conversion unit 109 converting the secret key of the user terminal device 100 by using a one-way function in a transform domain. Since the homomorphic encryption scheme used in the second embodiment is probabilistic, the encrypted distance contains a random number component. However, the random number component is no longer contained in the converted distance obtained by decryption using the converted key value.

2.2.2 Reference Converted Similarity Value Extraction Unit

The reference converted similarity value extraction unit 207 determines whether the converted distance matches the converted similarity value in the transform domain. If the converted distance matches any of converted similarity values, the reference converted similarity value extraction unit 207 determines that the converted distance satisfies a condition for the set converted similarity value and extracts the corresponding identifier ID. If the converted distance matches none of the converted similarity values, the reference converted similarity value extraction unit 207 does not extract the identifier ID.

2.3 Operation of Similarity-Information Search System

An operation of the similar-information search system 10 according to the second embodiment will be described with reference to a sequence diagram illustrated in FIG. 12.

FIG. 12 is a sequence diagram illustrating the operation of the similar-information search system 10 in accordance with the second embodiment.

As illustrated in FIG. 12, in step S101, the sensing unit 101 of the user terminal device 100 performs sensing on a user and obtains data about the user.

In step S102, the feature quantity calculation unit 102 calculates a feature quantity X from the information obtained by the sensing performed by the sensing unit 101. For example, the feature quantity X may be an m-dimensional vector whose components have following numerical values.

X=(x1, x2, . . . , xm)

In step S103, the similarity value setting unit 104 sets a similarity value that serves as a criterion during a search. For example, in the case where data for which the distance is less than D is searched for, distances 0, 1, 2, . . . , D−1 are set as the similarity values.

In step S104, the feature quantity encryption unit 103 encrypts the feature quantity X by using the homomorphic encryption scheme to generate an encrypted feature quantity. In addition, the similarity value conversion unit 110 determines a one-way function value of the similarity value. Specifically, the similarity value conversion unit 110 converts the similarity value by using the homomorphic encryption scheme to generate a converted similarity value. The homomorphic encryption scheme is an encryption scheme that makes decryption by the data computation device 200 unsuccessful. The key used during encryption is the public key of the user terminal device 100. Unlike the first embodiment, the homomorphic encryption scheme is a probabilistic encryption scheme containing a random number component in the second embodiment.

Let Enc1 denote an encryption function. Then, the encrypted feature quantity is denoted as

E1x=(Enc1(x1), Enc1(x2), . . . , Enc1(xm)).

Let “F” denote a one-way function used to convert the similarity value. Then, the converted similarity values can be denoted as

F(0), F(1), . . . , F(D−1).

In step S113, the key conversion unit 109 calculates a converted key value F(s), which is information obtained by converting a secret key s of the user terminal device 100 by using a certain function. The certain function is a one-way function. The key conversion unit 109 calculates the converted key value, which is a result of computation using the secret key s (e.g., result of exponentiation in which s is used as the exponent) when necessary.

In step S105, the user terminal device 100 requests the data computation device 200 to provide similar reference data that satisfies a condition, by providing the encrypted feature quantity, the converted similarity value, and the converted key value to the data computation device 200. That is, the user terminal device 100 transmits the encrypted feature quantity, the converted similarity value, and the converted key value to the data computation device 200, and the data computation device 200 receives these pieces of information. Specifically, the feature quantity encryption unit 103 transmits the encrypted feature quantity to the encrypted distance calculation unit 202, and the encrypted distance calculation unit 202 receives the encrypted feature quantity. In addition, the similarity value conversion unit 110 transmits the converted similarity value to the reference converted similarity value extraction unit 207, and the reference converted similarity value extraction unit 207 receives the converted similarity value. Further, the key conversion unit 109 transmits the converted key value to the converted distance decryption unit 206, and the converted distance decryption unit 206 receives the converted key value.

In addition, in step S105, the data computation device 200 may receive, as encrypted similarity values, a plurality of converted similarity values, which are obtained by encrypting a plurality of similarity values indicating a plurality of degrees of similarity by using a one-way function.

In step S106, the data computation device 200 selects the reference feature quantity Yi=(yi1, yi2, . . . , yim) assigned an i-th identifier ID from the reference feature quantity storage unit 201.

In step S107, the encrypted distance calculation unit 202 calculates a distance between the reference feature quantity encrypted using the encryption function Enc1 and the encrypted feature quantity, with the feature quantities kept encrypted. The encrypted reference feature quantity can be denoted as

E1y=(Enc1(yi1), Enc1(yi2), . . . , Enc1(yim)).

By using this equation, the encrypted distance calculation unit 202 can calculate, by determining differences between the corresponding vector components of the feature quantity X and the reference feature quantity Yi and adding the differences, by using the property of the homomorphic encryption scheme, the encrypted distance ED1i denoted as follows:

ED1i=Enc1(|x1−yi1|+|x2−yi2|+ . . . +|xm−yim|).

In the above, the sum of absolute values of differences between the corresponding vector components is used as the distance.

In step S114, the converted distance decryption unit 206 decrypts the encrypted distance by using the converted key value in the transform domain. As a result of the decryption,

F(|x1−yi1|+|x2−yi2|+ . . . +|xm−yim|)

is determined. This value is referred to as the converted distance. Note that the encrypted distance ED1i contains a random number generated at the time of encryption, and the outputs of the encryption function Enc1 do not necessarily match even if inputs are the same. In contrast, outputs of the one-way function F match if inputs are the same.

In step S108, the converted distance decryption unit 206 generates a converted distance by decrypting the encrypted distance by using the converted key value. In addition, the reference converted similarity value extraction unit 207 determines whether the generated converted distance matches the converted similarity value. Specifically, the reference converted similarity value extraction unit 207 determines whether the converted distance matches any of the converted similarity values F(0), F(1), . . . , F(D−1). If the converted distance matches any of the converted similarity values F(0), F(1), . . . , F(D−1), it indicates that content of the one-way function F, that is, the distance between the feature quantity and the reference feature quantity (|x1−yi1|+|x2−yi2|+ . . . , +|xm−yim|) is any of the distances 0, 1, . . . , D−1.

If the converted distance matches none of the converted similarity values F(0), F(1), . . . , F(D−1), the reference converted similarity value extraction unit 207 selects the next identifier ID, and the process returns to step S106.

If the converted distance matches any of the converted similarity values, the reference converted similarity value extraction unit 207 records the current identifier ID in step S109. Then, steps S106 to S109 are repeatedly performed for each identifier ID in the reference feature quantity storage unit 201.

In step S110, the encrypted reference data extraction unit 204 extracts reference data corresponding to each of the identifiers IDs extracted in step S109 from the reference data storage unit 205 and encrypts the reference data by using the public key of the user terminal device 100. Specifically, if it is determined in step S108 that the converted distance matches the converted similarity value, the encrypted reference data extraction unit 204 generates encrypted reference data, which is information obtained by encrypting reference data that is data corresponding to the reference feature quantity used in calculation of the encrypted distance.

In step S111, the encrypted reference data extraction unit 204 transmits the generated encrypted reference data to the user terminal device 100.

In step S112, the decryption unit 107 receives, from the data computation device 200, the encrypted reference data, which is information obtained by encrypting reference data that is obtained based on the encrypted feature quantity and the encrypted similarity value. The decryption unit 107 then decrypts the encrypted reference data that is transmitted from the encrypted reference data extraction unit 204 and is a result of the similar-information search, and the reference data utilization unit 106 utilizes the resulting reference data.

2.3.1 Specific Calculation Example for Ring Learning with Errors (LWE)

As a specific example of the encryption function Enc1, key generation, encryption, decryption, and multiplication of ciphertexts in a homomorphic encryption scheme using ring LWE will be described below. Ring LWE is described in chapter 13 of Shigeo Mitsunari, “Kuraudo wo Sasaeru Korekara no Ango Gijutsu (Applied Cryptography for the Cloud)”, Mar. 24, 2015, github.

Key generation: Rings R and Rp are formed by using n, which is a power to 2, and p, which is a prime number for which p−1 is a multiple of 2n. Let t denote a prime number smaller than p and Rt=(Z/tZ)[x]/(x̂n+1) be a plaintext space. Here, Z denotes an integer, and Rt denotes a residue field obtained by dividing by (x̂n+1) a polynomial whose coefficient is an integer smaller than or equal to t. In addition, the secret key s and e are arbitrarily selected, and a1 is arbitrarily selected from Rp. The public key is denoted by (a0=−(a1×s+t×e), a1).

Encryption: A plaintext m (∈Rt) is encrypted by using the public key (a0, a1) in a manner as follows:

Enc1(m)=(m+a0×e1+t×e3, a1×e1+t×e2), where e1, e2, and e3 are arbitrarily selected.

Decryption: A ciphertext c=(c0, c1, . . . , ck) is decrypted by using the secret key s in a manner as follows:

Dec1(c)=c0×ŝ0+c1×ŝ1+c2×ŝ2+ . . . +ck×ŝk (∈Rq).

Dec1(c) is m+t×e4 (where, |t×e4|<p/2).

Multiplication of ciphertexts: The product of the ciphertext c=(c0, c1, . . . , ck) and a ciphertext d=(d0, d1, . . . , dl) is denoted as h=(h0, h1, . . . , hk+1).

Here, hi is determined using

Σcj×di−j=c0×di+c1×di−1+ . . . ci×d0.

Addition of ciphertexts: The sum of the ciphertext c=(c0, c1, . . . , ck) and the ciphertext d=(d0, d1, . . . , dl) is determined by padding 0 to the shorter vector to make the lengths equal and then adding the corresponding components.

Specific calculations performed in the sequence illustrated in FIG. 12 will be described below using the encryption function Enc1 described above.

The encrypted feature quantity and the converted similarity values obtained in step S104 are respectively denoted as

E1x=(Enc1(x1), Enc1(x2), . . . , Enc1(xm)), and

F(0)=r̂0, F(1)=r̂1, F(D−1)=r̂D−1, where r is an arbitrary random number.

In step S113, the key conversion unit 109 determines the converted key value F(s) as follows:

F(s)=r̂s, F(ŝ2)=r̂(ŝ2), . . . , F(ŝk)=r̂(ŝk).

The encrypted reference feature quantity obtained in in step S106 is as follows:

E1y=(Enc1(y1), Enc1(y2), . . . , Enc1(ym)).

In step S107, the encrypted distance calculation unit 202 determines the encrypted distance ED1i by using the encrypted feature quantity E1x and the encrypted reference feature quantity E1y. Based on the homomorphic property of the encryption function Enc1 in terms of addition and multiplication,

ED1i=Enc1(distance between the feature quantity X and the reference feature quantity Yi)

is determined. Here, let the encrypted distance ED1i be (c0, c1, . . . , ck).

In step S114, the converted distance decryption unit 206 performs, by using the encrypted distance ED1i and the converted key value F(s), the following calculation.

r̂c0+F(s)̂c1+ . . . +F(ŝk)̂ck

From the above, this value is equal to r̂Dec1(ED1i). Dec1(ED1i), which is an exponent of this calculated value, is the distance between the feature quantity X and the reference feature quantity Yi.

In step S108, the value determined above is compared with each of the converted similarity values F(0)=r̂0, F(1)=r̂1, . . . , F(D−1)=r̂(D−1) (to determine whether they match). If the converted distance matches the converted similarity value, the distance between the feature quantity X and the reference feature quantity Yi is any of the distances 0 to (D−1). Thus, it is determined that the reference feature quantity satisfies the similarity condition. For the one-way function used to determine a power of r, outputs of the one-way function for different inputs may match (called collision) in some cases. However, it is assumed herein that such cases are ignorable in terms of probabilities.

2.3.2 Specific Calculation Example for Boneh-Goh-Nissim Cryptosystem (BGN Cryptosystem)

As another specific example of the encryption function Enc1, key generation, encryption, decryption, and multiplication of ciphertexts in BGN cryptosystem will be described below. BGN cryptosystem is described in D. Boneh, E.-J. Goh, K. Nissim, “Evaluating 2-DNF Formulas on Ciphertexts”, TCC05, 2005.

Key generation: N=q1×q2 is determined, where each of q1 and q2 is a prime number. Let each of G and G′ be a finite cyclic group of order of N, and a generator g of G and h=ûq2 (here, u is the generator of G) are generated. In addition, e denotes pairing of G×G→G′. Pairing is a function for which e(ûa, v̂b)=e(u,v)̂(ab) holds for arbitrary u, v∈G and a, b∈integers.

Among the values above, q1 is the secret key, and (N, e, g, h) is the public key.

Encryption: A plaintext m∈{0, 1, . . . , T} (T<q2) (this plaintext space is a space where a discrete logarithm problem can be solved) is encrypted by using the public key (N, e, g, h) in the following manner. The resulting ciphertext is denoted as

Enc1(m)=(ĝm)×(ĥr)=c, where r is an arbitrarily selected random number.

Decryption: The ciphertext c is decrypted by using the secret key q1 in the following manner.

${{Dec}\; 1(c)} = {{c\hat{}q}\; 1}$ $\begin{matrix} {{{Dec}\; 1(c)} = {{\left( {g\hat{}m} \right)\hat{}q}\; 1 \times {\left( {h\hat{}r} \right)\hat{}q}\; 1}} \\ {= {{\left( {{g\hat{}q}\; 1} \right)\hat{}m} \times {u\hat{}({Nr})}}} \\ {= {\left( {{g\hat{}q}\; 1} \right)\hat{}m}} \end{matrix}$

The discrete logarithm problem is solved for this equation to decrypt m.

Multiplication of ciphertexts: The multiplication homomorphic property is satisfied between the ciphertext E(m1) of m1 and the ciphertext E(m2) of m2 only once, and g1=e(g, g) and h1=e(g, h) hold.

If u=ĝα, h=ûq2=ĝαq2 is obtained. At that time, a ciphertext for m1m2 can be created from E1(m) and E1(m2) in the following manner.

E1(m1)=(ĝm1)×(ĥr1)

E1(m2)=(ĝm2)×(ĥr2)

e(E1(m1), E1(m2))h1̂r=(g1̂(m1m2))h1̂r3∈G′

Here, r3=m1r2+r2m1+αq2r1r2+r is obtained.

Addition of ciphertexts: The following addition homomorphic property is satisfied between the ciphertext E1(m1) of m1 and the ciphertext E1(m2) of m2. That is, ciphertext for m1+m2 can be created from the ciphertext E1(m1) and the ciphertext E1(m2).

E1(m1)×E1(m2)=(ĝ(m1+m2))×ĥ(r1+r2)=E1(m1+m2).

Specific calculations performed in the sequence illustrated in FIG. 12 will be described below using the encryption function Enc1 described above.

The encrypted feature quantity and the converted similarity values obtained in step S104 are respectively denoted as

E1x=(Enc1(x1), Enc1(x2), . . . , Enc1(xm)), and

F(0)=g1̂Q, F(1)=g1̂2Q, . . . , F(D−1)=g1̂((D−1)Q), where r is an arbitrary random number.

In step S113, the key conversion unit 109 generates an arbitrary random number R and determines the converted key value F(s), which is denoted as

F(s)=R×q1=Q.

The encrypted reference feature quantity obtained in step S106 is denoted as follows:

E1y=(Enc1(y1), Enc1(y2), . . . , Enc1(ym)).

In step S107, the encrypted distance calculation unit 202 determines the encrypted distance ED1i by using the encrypted feature quantity E1x and the encrypted reference feature quantity E1y. Based on the homomorphic property of the encryption function Enc1 in terms of addition and multiplication, an equation

ED1i=Enc1(distance between x and y)

is satisfied. Note that the distance between x and y may be, for example, a difference therebetween or a cosine similarity (x1y1+x2y2+ . . . +xmym) by using the multiplication homomorphic property that holds only once.

Note that ED1i takes a form of (g1̂(distance between x and y))×(ĥr4) and contains a random number component.

In step S114, the following calculation is performed by using the encrypted distance ED1i and the converted key value.

$\begin{matrix} {{\left( {{ED}\; 1\; i} \right)\hat{}{F(s)}} = {\left( {\left( {g\; {1\hat{}\left( {{distance}\mspace{14mu} {between}\mspace{14mu} x\mspace{14mu} {and}\mspace{14mu} y} \right)}} \right) \times \left( {{h\hat{}r}\; 4} \right)} \right)\hat{}\left( {R \times q\; 1} \right)}} \\ {= {\left( {g\; {1\hat{}\left( {{distance}\mspace{14mu} {between}\mspace{14mu} x\mspace{14mu} {and}\mspace{14mu} y} \right)}} \right)\hat{}Q}} \\ {= {\left( {g\; {1\hat{}Q}} \right)\hat{}\left( {{distance}\mspace{14mu} {between}\mspace{14mu} x\mspace{14mu} {and}\mspace{14mu} y} \right)}} \end{matrix}$

In step S108, the converted distance decryption unit 206 compares the value determined above with each of the converted similarity values F(0)=g1̂Q, F(1)=g1̂(2Q), . . . , F(D−1)=g1̂((D−1)Q). If the converted distance matches any of the converted similarity values, the distance between the feature quantity X and the reference feature quantity Yi is any of 0 to (D−1). Thus, it is determined that the reference feature quantity satisfies the similarity condition.

2.4 Advantageous Effects of Second Embodiment

In the second embodiment, since the user terminal device 100 transmits data to the data computation device 200 by using probabilistic encryption containing a random number, the security can be enhanced. In addition, the data computation device 200 receives the encrypted feature quantity, the encrypted similarity value, and the converted key value from the user terminal device 100 and extracts encrypted reference data. Since the data computation device 200 extracts only similar data that satisfies the similarity value less than D and transmits the similar data to the user terminal device 100, an amount of calculation performed by the user terminal device 100 and an amount of data communicated between the data computation device 200 and the user terminal device 100 can be reduced. In addition, the user terminal device 100 can flexibly set the similarity value in accordance with a user's desire.

With the probabilistic encryption containing a random number, the encrypted feature quantities do not necessarily have the same value even if the user performs a search using the same information. Accordingly, this similarity-information search method or the like can enhance the confidentiality of the information searched for by the user during transmission from the user terminal device 100 to the data computation device 200 and in the data computation device 200. As a result, the use of the probabilistic encryption makes an analysis based on a frequency in ciphertexts more difficult and can enhance the security.

Third Embodiment

A similar-information search system relating to the present disclosure will now be described as a third embodiment of the present disclosure with reference to the drawings. Differences between the third embodiment and the first and second embodiments are as follows. In the first and second embodiments, the feature quantity determined by the user terminal device 100 relates to user's privacy, and a similar-information search is performed with the feature quantity kept confidential from the data computation device 200 by encryption. However, some users may wish to obtain a more detailed search result by disclosing their feature quantities to the data computation device 200. Some users may wish to accumulate their feature quantities in the reference feature quantity storage unit 201 and the reference data storage unit 205 and make use of their feature quantities as the reference data during a similar-information search performed by other users. In the third embodiment, a user's desire is flexibly coped with by switching the encryption scheme and whether or not to accumulate the feature quantity in the reference feature quantity storage unit 201 in accordance with a selection made by the user. Hereinafter, these differences will be mainly described, and a description about components having substantially the same function is omitted.

3.1 User Terminal Device

FIG. 14 is a block diagram illustrating a configuration of the user terminal device 100 in accordance with the third embodiment.

As illustrated in FIG. 14, the user terminal device 100 further includes an encryption switching unit 111 and an instruction transmission unit 112 in addition to the components of the first embodiment. Since components, in the block diagram of FIG. 14, that are denoted by the same reference signs as those used in FIG. 3 have substantially the same functions as those in FIG. 3, and thus a description thereof is omitted. The feature quantity encryption unit 103, the similarity value encryption unit 105, the decryption unit 107, and the instruction transmission unit 112 are connected to corresponding components (indicated by A, B, C, and H, respectively) in FIG. 15 (described later).

3.1.1 Encryption Switching Unit

The encryption switching unit 111 obtains, from a user, an instruction indicating which of an encrypted search and an unencrypted search is to be performed. In the encrypted search, the data computation device 200 performs a search using information provided by the user terminal device 100 in an encrypted state. In the unencrypted search, the data computation device 200 performs a search using information provided by the user terminal device 100 in an unencrypted state.

That is, the encryption switching unit 111 switches between whether to keep the feature quantity confidential from the data computation device 200 in accordance with a user's desire. As a specific example of this switching, the encryption switching unit 111 switches the key used by the feature quantity encryption unit 103 between the public key of the user terminal device 100 and the public key of the data computation device 200. If the public key of the user terminal device 100 is used, only the user terminal device 100 has the corresponding secret key. Thus, the data computation device 200 performs a similar-information search while keeping the feature quantity encrypted. On the other hand, if the public key of the data computation device 200 is used, the data computation device 200 has the corresponding secret key. Thus, the data computation device 200 decrypts the encrypted feature quantity and performs a similar-information search using the resulting plaintext. As another specific example of the switching, the encryption switching unit 111 switches between a setting for using the public key of the user terminal device 100 as the key used by the feature quantity encryption unit 103 or a setting for not performing encryption.

If the encryption switching unit 111 obtains an instruction to perform an encrypted search, a homomorphic encryption scheme that makes decryption by the data computation device 200 unsuccessful is used. If the encryption switching unit 111 obtains an instruction to perform an unencrypted search, an encryption scheme that makes decryption by the data computation device 200 successful is used.

Note that if encryption is performed using the public key of the data computation device 200 or if a similar-information search is requested without encryption, the data computation device 200 is permitted to select whether to store the user's case and log information in the reference feature quantity storage unit 201 and the reference data storage unit 205 in accordance with a user's desire.

FIG. 17 is a diagram illustrating an example of a data format used by the user terminal device 100 and the data computation device 200 in accordance with the third embodiment.

As illustrated in FIG. 17, if the user selects to store their case and log information in the reference feature quantity storage unit 201 and the reference data storage unit 205, the user terminal device 100 notifies the data computation device 200 of this fact by using a storage permission flag. It is necessary that information of this storage permission flag be not tampered. For example, a message authentication code or a digital signature may be attached.

3.1.2 Instruction Transmission Unit

The instruction transmission unit 112 transmits the instruction obtained by the encryption switching unit 111 to the data computation device 200. The instruction transmission unit 112 notifies the data computation device 200 of the information regarding the switching made by the instruction transmission unit 112 by using an encryption flag field as illustrated in FIG. 17.

3.1.3 Decryption Unit

The decryption unit 107 receives, from the data computation device 200, encrypted reference data, which is information obtained by encrypting reference data (similar-information search result) obtained based on the encrypted feature quantity and the encrypted similarity value through an encrypted search or an unencrypted search, which is performed in accordance with the instruction obtained by the encryption switching unit 111. The decryption unit 107 then decrypts the received encrypted reference data.

3.2 Data Computation Device

FIG. 15 is a block diagram illustrating a configuration of the data computation device 200 in accordance with the third embodiment.

As illustrated in FIG. 15, the data computation device 200 further includes an encryption control unit 208 in addition to the components of the first embodiment. Note that components, in the block diagram of FIG. 15, that are denoted by the same reference signs as those used in FIG. 4 have substantially the same functions as those in FIG. 4, and thus a description thereof is omitted.

3.2.1 Encryption Control Unit

The encryption control unit 208 obtains the instruction transmitted from the instruction transmission unit 112. The encryption control unit 208 refers to the encryption flag and the storage permission flag illustrated in FIG. 17 and performs control corresponding to the switching according to the user's desire. When the encryption flag is 0, the encryption control unit 208 performs a similar-information search while keeping information encrypted. On the other hand, when the encryption flag is 1, the encryption control unit 208 decrypts the feature quantity by using the secret key of the data computation device 200 and performs a similar-information search using the resulting plaintext. Alternatively, the encryption control unit 208 performs a similar-information search using the transmitted plaintext feature quantity. In addition, if the storage permission flag is 1, the encryption control unit 208 stores the information about the user in the reference feature quantity storage unit 201 and the reference data storage unit 205.

3.3 Operation of Similar-Information Search System

An operation of the similar-information search system 10 will be described with reference to FIG. 16.

FIG. 16 is a sequence diagram illustrating the operation of the similar-information search system 10 in accordance with the third embodiment.

As illustrated in FIG. 16, in step S101, the sensing unit 101 of the user terminal device 100 performs sensing on a user and obtains information about the user.

In step S115, the user terminal device 100 obtains an instruction indicating which of an encrypted search, in which the data computation device 200 performs a search using the information provided by the user terminal device 100 in an encrypted state, and an unencrypted search, in which the data computation device 200 performs a search using the information provided by the user terminal device 100 in an unencrypted state, is to be performed.

In step S102, the feature quantity calculation unit 102 calculates a feature quantity X from the information obtained by the sensing. For example, the feature quantity X may be an m-dimensional vector whose components have following numerical values.

X=(x1, x2, . . . , xm)

In step S103, the similarity value setting unit 104 sets a similarity value that serves as a criterion during a search. For example, in the case where data for which the distance is less than D is searched for, distances 0, 1, 2, . . . , D−1 are set as the similarity values.

In step S104, the feature quantity encryption unit 103 encrypts the feature quantity calculated by the feature quantity calculation unit 102 by using an encryption scheme dependent on the instruction obtained in step S115 to generate an encrypted feature quantity. The similarity value encryption unit 105 encrypts the similarity value, indicating a degree of similarity, using the encryption scheme dependent on the instruction obtained in step S115 to generate an encrypted similarity value. If an instruction to perform an encrypted search is obtained in step S115, a homomorphic encryption scheme that makes decryption by the data computation device 200 unsuccessful is used. If an instruction to perform an unencrypted search is obtained in step S115, an encryption scheme that makes decryption by the data computation device 200 successful is used. In the case where homomorphic encryption is used as the encryption scheme, the public key of the user terminal device 100 is used.

Let Enc denote an encryption function. Then, the encrypted feature quantity is denoted as

Ex=(Enc(x1), Enc(x2), . . . , Enc(xm)), and

the encrypted similarity values can be denoted as

Enc(0), Enc(1), . . . , Enc(D−1).

In step S105, the user terminal device 100 requests the data computation device 200 to provide similar reference data that satisfies a condition, by providing the encrypted feature quantity and the encrypted similarity value to the data computation device 200 if the user terminal device 100 has obtained an instruction to perform an encrypted search. Specifically, the user terminal device 100 transmits the instruction obtained in step S115 to the data computation device 200.

In step S104, the feature quantity is encrypted by using the public key corresponding to the secret key of the user terminal device 100 as the encryption scheme if an instruction to perform an encrypted search has been obtained in step S115. In addition, in step S104, the feature quantity is encrypted by using the public key corresponding to the secret key of the data computation device 200 as the encryption scheme if an instruction to perform an unencrypted search has been obtained in step S115.

In step S104, the similarity value is encrypted by using the public key corresponding to the secret key of the user terminal device 100 as the encryption scheme if an instruction to perform an encrypted search has been obtained in step S115. In addition, in step S104, the similarity value is encrypted by using the public key corresponding to the secret key of the data computation device 200 as the encryption scheme if an instruction to perform an unencrypted search has been obtained in step S115.

In step S106, the data computation device 200 selects the reference feature quantity Yi=(yi1, yi2, . . . , yim) assigned an i-th identifier ID from the reference feature quantity storage unit 201.

In step S116, the encryption control unit 208 receives the instruction transmitted from the user terminal device 100. The encryption control unit 208 determines which of an encrypted search or an unencrypted search is to be performed in accordance with the received instruction.

In step S107, the encrypted distance calculation unit 202 calculates an encrypted distance, which indicates a degree of similarity between the encrypted feature quantity received from the user terminal device 100 and an encrypted reference feature quantity obtained by encrypting at least one of the plurality reference feature quantities, if it is determined in step S116 that an encrypted search is to be performed. Also, if it is determined in step S116 that an unencrypted search is to be performed, the encrypted distance calculation unit 202 calculates a distance, which indicates a degree of similarity between a decrypted feature quantity obtained by decrypting the encrypted feature quantity received from the user terminal device 100 and at least one of the plurality of reference feature quantities. Note that the encrypted reference feature quantity obtained by encrypting at least one of the plurality of reference feature quantities may be replaced with an unencrypted reference feature quantity.

Specifically, in step S107, the encrypted distance calculation unit 202 calculates the distance between the encrypted feature quantity and the reference feature quantity encrypted by the encryption function Enc. The encrypted reference feature quantity can be denoted as

Ey=(Enc(yi1), Enc(yi2), . . . , Enc(yim)).

The encrypted distance calculation unit 202 calculates, by determining differences between the corresponding vector components of the feature quantity X and the reference feature quantity Yi and adding the differences, by using the property of the homomorphic encryption scheme, the encrypted distance EDi as follows:

EDi=Enc(|x1−yi1|+|x2−yi2|+ . . . +|xm−yim|).

In the above, the sum of absolute values of differences between the corresponding vector components is used as the encrypted distance.

In step S108, the reference similarity value extraction unit 203 determines whether the encrypted distance calculated in step S107 matches the encrypted similarity value received from the user terminal device 100 if it is determined in step S116 that an encrypted search is to be performed. On the other hand, if it is determined in step S116 that an unencrypted search is to be performed, the reference similarity value extraction unit 203 determines whether the distance calculated in step S107 matches the decrypted similarity value obtained by decrypting the encrypted similarity value received from the user terminal device 100.

Specifically, in step S108, the reference similarity value extraction unit 203 checks whether a condition is satisfied by using the encrypted distance and the encrypted similarity value. For example, the reference similarity value extraction unit 203 checks whether the encrypted distance EDi matches any of the encrypted similarity values Enc(0), Enc(1), . . . , Enc(D−1). If the encrypted distance EDi matches any of the encrypted similarity values, it means that content of the encryption function Enc for Edi, i.e., the distance between the feature quantity and the reference feature quantity (|x1−yi1|+|x2−yi2|+ . . . +|xm−yim|) is any of 0, 1, . . . , D−1.

If the encrypted distance EDi matches none of the encrypted similarity values, the next identifier ID is selected. The process then returns to step S106.

If the encrypted distance EDi matches any of the encrypted similarity values, it indicates that the distance between the feature quantities is smaller than the distance D. Thus, the current identifier ID is recorded in step S109. Steps S106 to S109 are repeatedly performed for each identifier ID in the reference feature quantity storage unit 201.

In step S110, the encrypted reference data extraction unit 204 extracts reference data corresponding to each of the identifiers IDs extracted in step S109 from the reference data storage unit 205 and encrypts the reference data by using the public key of the user terminal device 100. Specifically, if it is determined in step S108 that the encrypted distance matches the encrypted similarity value, the encrypted reference data extraction unit 204 generates encrypted reference data, which is information obtained by encrypting reference data that is data corresponding to the reference feature quantity used in calculation of the encrypted distance or the distance.

In step S111, the encrypted reference data extraction unit 204 transmits the generated encrypted reference data to the user terminal device 100.

In step S112, the user terminal device 100 decrypts and utilizes the reference data. Specifically, the data computation device 200 performs an encrypted search or an unencrypted search in accordance with the instruction obtained in step S115. The decryption unit 107 receives, from the data computation device 200, the encrypted reference data, which is information obtained by encrypting reference data obtained based on the encrypted feature quantity and the encrypted similarity value through the encrypted search or the unencrypted search. The decryption unit 107 then decrypts the encrypted reference data transmitted from the encrypted reference data extraction unit 204.

3.4 Display Example in Similar-Information Search System

FIG. 18 is a diagram illustrating an example of a screen displayed on the user terminal device 100 in accordance with the third embodiment.

As illustrated in FIG. 18, in this example, when the user clicks “Start” displayed in the screen on the user terminal device 100, the sensing unit 101 starts measuring blood pressure. Then, the feature quantity calculation unit 102 calculates a feature quantity from a tendency by using the measured value and the recent log information. Before the user searches for similar cases, the user selects one of

-   A) Perform similar-information search with information about you     kept encrypted, -   B) Perform similar-information search with information about you     disclosed to the server, and -   C) Cancel search. Here, A) and B) are an example of step S115.

If the user selects A), the encryption switching unit 111 illustrated in FIG. 14 makes a setting so that encryption is performed using the public key of the user terminal device 100, and the data computation device 200 performs a similar-information search, with the information kept encrypted.

If the user selects B), the encryption switching unit 111 illustrated in FIG. 14 makes a setting so that encryption is performed using the public key of the data computation device 200 or any encryption is not performed, and the data computation device 200 performs a similar-information search using a plaintext.

If the user selects C), the search is canceled.

In the case of A) and B), the user terminal device 100 transmits, to the data computation device 200, the storage permission flag that indicates whether to permit registration of information about the user as information used in an encrypted search or an unencrypted search in the data computation device 200.

In response to selection of B), for example, a screen illustrated in FIG. 19 is displayed.

FIG. 19 is a diagram illustrating an example of a screen displayed on the user terminal device 100 in accordance with the third embodiment.

As illustrated in FIG. 19, the user selects, in this screen, whether to permit accumulation of information about them in the database of the server. If the user selects “Permit”, the information about the user is accumulated in the reference feature quantity storage unit 201 and the reference data storage unit 205. That is, the data computation device 200 registers the information about the user transmitted from the user terminal device 100 in the reference data storage unit 205 as at least one of the reference feature quantity and the reference data in response to the user instruction, in accordance with the storage permission flag received from the user terminal device 100. The information about the user includes not only information obtained by sensing performed by the sensing unit 101 of the user terminal device 100 but also information about advice provided to the user and information about advice selected by the user. In addition, the user may transmit the information about them such as a medical history to the data computation device 200 by using a general-purpose device such as a personal computer.

3.5 Advantageous Effects of Third Embodiment

In third embodiment, the user can switch between a similar-information search performed with the user's feature quantity kept encrypted or a similar-information search performed using a plaintext. Some users may wish to obtain a more detailed search result by disclosing their feature quantities to the data computation device 200. Some users may wish to accumulate their feature quantities in the reference feature quantity storage unit 201 and the reference data storage unit 205 and make use of their feature quantities as the reference data during a similar-information search performed by other users. That is, the user can select whether to provide information about them to an external device (e.g., database of the server) via the data computation device 200. Accordingly, in a similar-information search method, a terminal device, and a similar-information search system, an operation can be flexibly performed in accordance with a user's desire.

In addition, the feature quantity is encrypted, and the data computation device 200 is unable to decrypt the encrypted feature quantity. Accordingly, this similarity-information search method or the like can enhance the confidentiality of the information searched for by the user over a transmission/reception channel between the user terminal device 100 and the data computation device 200 and in the data computation device 200.

Modifications

While the present disclosure has been described on the basis of the first to third embodiments, it is obvious that the present disclosure is not limited to the first to third embodiments. The following cases are also included in the present disclosure.

(1) The description has been given on the assumption that the feature quantity is a vector in the first and second embodiments; however, the feature quantity may be a numerical value. In addition, this numerical value may be mapped.

(2) Consecutive values smaller than D are set as the similarity values in the first and second embodiments; however, for example, a value indicating complete matching (that is, zero) or discrete values may be set as the similarity values.

(3) FIG. 13 is a block diagram illustrating a configuration of the data computation device 200 in the case where statistical reference data is obtained in accordance with a modification.

As illustrated in FIG. 13, the data computation device 200 additionally includes an encrypted reference data statistics calculation unit 209 that determines a statistical value. In the first and second embodiments, the data computation device 200 transmits the reference data to the user terminal device 100 after encrypting the reference data; however, the data computation device 200 may derive frequency information or a statistical value, for example, an average or a variance from the reference data and transmit the frequency information or the statistical value to the user terminal device 100. In this case, providing the statistical value instead of raw reference data to the user terminal device 100 makes it difficult to identify the user who owns the reference data even if the reference data leaks to the outside. Accordingly, in this configuration, privacy protection is enhanced.

(4) In the first and second embodiments, plaintext data is stored in the reference feature quantity storage unit 201 and the reference data storage unit 205 of the data computation device 200; however, the data may be encrypted. In this case, the data can be used by extracting the data from the reference data storage unit 205 and decrypting the data. As a result, the security is enhanced. In addition, the data computation device 200 may store data that is encrypted by using the public key of the user terminal device 100. In this case, the reference data can be used in calculation of the encrypted distance or transmitted to the user terminal device 100 without processing the reference data.

(5) In the first and second embodiments, the public key and the secret key of the user terminal device 100 are stored in the key storage unit 108; however, the public key and the secret key may be stored on another medium that is separate from the user terminal device 100 and is attachable to the user terminal device 100 when the user utilizes the user terminal device 100, for example.

(6) In the first and second embodiments, the public key and the secret key of the user terminal device 100 are keys of the user terminal device 100; however, keys for each user may be stored in the key storage unit 108. In this case, authentication is performed between the user and the user terminal device 100. If the authentication is successful, the key corresponding to the user can be selected from the key storage unit 108 and utilized. In this case, in the example data format illustrated in FIGS. 7A and 7B, an identifier ID of the user is used in place of the identifier ID of the user terminal device 100.

(7) In the first and second embodiments, the feature quantity calculation unit 102 may include therein a database, accumulate, for each user, information obtained by sensing, and calculate a feature quantity from information obtained by sensing in the past. In this case, the user may be identified as a result of the user inputting their identifier ID and the user terminal device 100 authenticating the user, for example.

(8) In the first and second embodiments, the feature quantity calculation unit 102 of the user terminal device 100 may obtain, from a database in another medium, information obtained by sensing in the past and calculate the feature quantity. In this case, the identifier ID of the user may be stored on the other medium, and the user terminal device 100 may read it.

(9) In the first and second embodiments, the data computation device 200 searches for reference data (for example, a medical history or treatment history in this case) of users who have the similar tendency on the basis of a feature quantity of the vital signs, such as blood pressure; however, the feature quantity and the reference data are not limited to this type of data. For example, a feature quantity may be calculated from data indicating a range of activities by using a global positioning system (GPS), and reference data (for example, a purchase history or a history about places to eat and drink) of users who have the similar tendency may be searched for on the basis of this feature quantity.

(10) In the first and second embodiments, the user terminal device 100 transmits the similarity value to the data computation device 200 after encrypting the similarity value; however, this encryption may be performed by the data computation device 200. That is, the similarity value encryption unit 105 of the user terminal device 100 may be included in the data computation device 200. Note that the similarity value need not be encrypted if there is no need to keep the similarity value confidential.

(11) In the first and second embodiments, the data computation device 200 includes the reference data storage unit 205; however, another device different from the data computation device 200 may include the reference data storage unit 205. In this case, the other device different from the data computation device 200 may be notified of the identifier ID that is extracted based on the feature quantity and the similarity value, and the other device may calculate the corresponding reference data.

(12) In the first and second embodiments, every reference data corresponding to each of the identifiers IDs and extracted as data similar to the feature quantity is transmitted to the user terminal device 100; however, every reference data is not necessarily transmitted all the time. That is, the data computation device 200 may select part of the reference data and transmit the selected part of the reference data to the user terminal device 100.

In addition, as illustrated in FIGS. 6 and 12, the data computation device 200 checks, for all the identifiers IDs in the reference data storage unit 205, whether the feature quantity is similar to the reference data by using the similarity values of 0 to D−1; however, the configuration is not limited to this one. For example, a method (nesting) for sequentially performing the check may be used in which the check is first performed for all the identifiers IDs using the similarity value of 0, and then performed using the similarity value of 1, and ultimately performed using the similarity value of D−1. If the distance matches any of the similarity values during the check, the corresponding identifier ID is extracted.

Further, for example, a method may be used in which the data computation device 200 predetermines the maximum number of pieces of encrypted reference data transmitted to the user terminal device 100, stops repeating the check upon the predetermined number being reached while the data computation device 200 is repeatedly performing the check, extracts and encrypts pieces of reference data corresponding to the extracted identifiers IDs, and returns the encrypted pieces of reference data to the user terminal device 100.

(13) As illustrated in FIG. 18, in the third embodiment, the similar-information search result displayed on the user terminal device 100 does not change in the case of A) where the similar-information search is performed with the user's feature quantity kept encrypted and in the case of B) where the similar-information search is performed using a plaintext by disclosing the user's feature quantity to the server. However, the user terminal device 100 may display only the statistical data in the case of A) and display more detailed individual information in the case of B).

(14) As illustrated in FIG. 19, in the third embodiment, when the user permits accumulation of their information in the database via the user terminal device 100, the user may be given an incentive such as reward points. In addition, when the user permits accumulation of their information in the database via the user terminal device 100, an agreement about how to handle the information (such as a consent agreement or an agreement about privacy) may be displayed to obtain an agreement from the user.

(15) In the third embodiment, the data obtained by sensing performed by the sensing unit 101 on the user is blood pressure data; however, in the case where there are a plurality of items to be measured, such as blood pressure information and weight information, the user may select, for each item to be measured, whether to permit accumulation of the information in the database.

(16) As illustrated in FIG. 18, in the third embodiment, the public key used is switched in the case of A) where the similar-information search is performed with the user's feature quantity kept encrypted and in the case of B) where the similar-information search is performed using a plaintext by disclosing the user's feature quantity to the data computation device 200. In the case of A), encryption may be performed by using the public key of the user terminal device 100, and in the case of B), encryption may be performed by using the public key of the data computation device 200. In addition to switching of the public key, the encryption scheme may also be switched.

Note that the techniques described in the above embodiments and modifications can be implemented by using a cloud service of the following types, for example. However, the types of the cloud service that implements the techniques described in the above embodiments and modifications are not limited to these ones.

Service Type 1: Local-Data-Center-Based Cloud Service

FIG. 20 is a block diagram illustrating an overview of a service provided by the similar-information search system of a service type 1 (local-data-center-based cloud service).

As illustrated in FIG. 20, in this type, the service provider 1120 obtains information from the group 1000 and provides a service to the user. In this type, the service provider 1120 functions as the data center operating company. That is, the service provider 1120 has a data center (cloud server) 1203 that manages big data. Accordingly, there is no data center operating company.

In this type, the service provider 1120 operates and manages the data center (cloud server) 1203. In addition, the service provider 1120 manages an operating system (OS) 1202 and an application 1201. The service provider 1120 provides a service by using the OS 1202 and the application 1201 managed thereby (arrow 1204).

Service Type 2: IaaS-Based Cloud Service

FIG. 21 is a block diagram illustrating an overview of a service provided by the similar-information search system of a service type 2 (IaaS-based cloud service). The term “IaaS (infrastructure as a service)” refers to a cloud service providing model in which an infrastructure on which a computer system is constructed and operated is provided as a service via the Internet.

As illustrated in FIG. 21, in this type, the data center operating company 1110 operates and manages the data center (cloud server) 1203. In addition, the service provider 1120 manages the OS 1202 and the application 1201. The service provider 1120 provides the service by using the OS 1202 and the application 1201 managed thereby (arrow 1204).

Service Type 3: PaaS-Based Cloud Service

FIG. 22 is a block diagram illustrating an overview of a service provided by the similar-information search system of a service type 3 (PaaS-based cloud service). The term “PaaS (platform as a service)” refers to a cloud service providing model in which a platform serving as a foundation for constructing and operating software is provided as a service via the Internet.

As illustrated in FIG. 22, in this type, the data center operating company 1110 manages the OS 1202 and operates and manages the data center (cloud server) 1203. In addition, the service provider 1120 manages the application 1201. The service provider 1120 provides the service by using the OS 1202 managed by the data center operating company 1110 and the application 1201 managed by the service provider 1120 (arrow 1204).

Service Type 4: SaaS-Based Cloud Service

FIG. 23 is a block diagram illustrating an overview of a service provided by the similar-information search system of a service type 4 (SaaS-based cloud service). The term “SaaS (software as a service)” refers to a cloud service providing model having a function that enables a user such as a company or person not owning the data center (cloud server) 1203 to use, via a network such as the Internet, the application 1201 provided by a platform provider that owns the data center (cloud server) 1203, for example.

As illustrated in FIG. 23, in this type, the data center operating company 1110 manages the application 1201 and the OS 1202 and operates and manages the data center (cloud server) 1203. In addition, the service provider 1120 provides the service using the OS 1202 and the application 1201 managed by the data center operating company 1110 (arrow 1204).

In any of the cloud service types 1 to 4 described above, the service provider 1120 provides the service. In addition, the service provider 1120 or the data center operating company 1110 may develop an OS, an application, or a database storing big data by itself, or let a third party do the development.

Embodiments obtained by making various modifications conceivable by a person skilled in the art on the first to third embodiments, the modifications, and the service types 1 to 4 or embodiments implemented by combining the elements and functions of the first to third embodiments, the modifications, and the service types 1 to 4 in a given manner within a scope not departing from the essence of the present disclosure are also included in the present disclosure.

The similar-information search method according to an aspect of the present disclosure allows a similar-information search to be performed, with content of a feature quantity kept confidential by encryption, and is applicable to a computer system that performs a similar-information search for cases of diseases, for example. 

What is claimed is:
 1. A method comprising: a terminal device obtaining an instruction indicating which of an encrypted search, in which a server device performs the search using the information provided by the terminal device in an encrypted state, and an unencrypted search, in which the server device performs the search using the information provided by the terminal device in an unencrypted state, is to be performed; the terminal device generating a first encrypted feature quantity by encrypting, using an encryption scheme dependent on the obtained instruction, a first feature quantity indicating a feature of obtained data, the encryption scheme dependent on the obtained instruction being a first encryption scheme that is homomorphic and makes decryption by the server device unsuccessful in a case where the instruction is an instruction to perform an encrypted search and being a second encryption scheme that makes decryption by the server device successful in a case where the instruction is an instruction to perform an unencrypted search; the terminal device transmitting the generated first encrypted feature quantity to the server device; the terminal device generating an encrypted similarity value by encrypting, using the encryption scheme dependent on the instruction, a similarity value indicating a degree of similarity; the terminal device transmitting the generated encrypted similarity value to the server device; the terminal device transmitting the obtained instruction to the server device; the terminal device receiving encrypted reference data from the server device, the encrypted reference data being information obtained by encrypting reference data that is obtained as a result of the server device searching for similar information in accordance with the instruction; and the terminal device decrypting the received encrypted reference data.
 2. The method according to claim 1, the server device including a first memory that stores a plurality of reference feature quantities and a second memory that stores a plurality of pieces of reference data each corresponding to one of the plurality of reference feature quantities, further comprising: the server device receiving the instruction transmitted from the terminal device; the server device deciding which of the encrypted search and the unencrypted search is to be performed in accordance with the received instruction; the server device calculating an encrypted distance indicating a degree of similarity between the first encrypted feature quantity received from the terminal device and a first reference feature quantity that is one of the plurality of reference feature quantities stored in the first memory, in a case where it is decided in the deciding that the encrypted search is to be performed; the server device determining whether the calculated encrypted distance matches the encrypted similarity value received from the terminal device; the server device obtaining first reference data from the second memory in a case where it is determined in the determining that the calculated encrypted distance matches the encrypted similarity value, the first reference data being data corresponding to the first reference feature quantity used in the calculating of the encrypted distance; the server device generating the encrypted reference data by encrypting the obtained first reference data; and the server device transmitting the generated encrypted reference data to the terminal device.
 3. The method according to claim 1, the server device including a first memory that stores a plurality of reference feature quantities and a second memory that stores a plurality of pieces of reference data each corresponding to one of the plurality of reference feature quantities, further comprising: the server device receiving the instruction transmitted from the terminal device; the server device deciding which of the encrypted search and the unencrypted search is to be performed in accordance with the received instruction; the server device calculating a distance indicating a degree of similarity between a decrypted feature quantity obtained by decrypting the first encrypted feature quantity received from the terminal device and a first reference feature quantity that is one of the plurality of reference feature quantities, in a case where it is decided in the deciding that the unencrypted search is to be performed; the server device determining whether the calculated distance matches a decrypted similarity value obtained by decrypting the encrypted similarity value received from the terminal device; the server device obtaining first reference data from the second memory, the first reference data being data corresponding to the first reference feature quantity used in the calculating of the distance in a case where it is determined in the determining that the calculated distance matches the decrypted similarity value; the server device generating the encrypted reference data by encrypting the obtained first reference data; and the server device transmitting the generated encrypted reference data to the terminal device.
 4. The method according to claim 1, wherein the first encryption scheme is an encryption scheme in which encryption is performed using a public key corresponding to a secret key of the terminal device, and wherein the second encryption scheme is an encryption scheme in which encryption is performed using a public key corresponding to a secret key of the server device.
 5. The method according to claim 1, further comprising: the terminal device transmitting registration permission information to the server device, the registration permission information being information indicating whether registration of information about a user of the terminal device in the server device is permitted; and the server device registering the information about the user transmitted from the terminal device, as at least one of the reference feature quantity and the reference data in a case where the received registration permission information indicates that registration is permitted.
 6. A terminal device comprising: one or more memories; and circuitry which, in operation,: obtains an instruction indicating which of an encrypted search, in which a server device performs the search using information provided by the terminal device in an encrypted state, and an unencrypted search, in which the server device performs the search using the information provided by the terminal device in an unencrypted state, is to be performed; generates a first encrypted feature quantity by encrypting, using an encryption scheme dependent on the obtained instruction, a first feature quantity indicating a feature of obtained data, the encryption scheme dependent on the obtained instruction being a first encryption scheme that is homomorphic and makes decryption by the server device unsuccessful in a case where the instruction is an instruction to perform an encrypted search and being a second encryption scheme that makes decryption by the server device successful in a case where the instruction is an instruction to perform an unencrypted search; transmits the generated first encrypted feature quantity to the server device; generates an encrypted similarity value by encrypting, using the encryption scheme dependent on the instruction, a similarity value indicating a degree of similarity; transmits the generated encrypted similarity value to the server device; transmits the obtained instruction to the server device; receives encrypted reference data from the server device, the encrypted reference data being information obtained by encrypting reference data that is obtained as a result of the server device searching for similar information in accordance with the instruction; and decrypts the received encrypted reference data.
 7. A system comprising: a terminal device that provides information; and a server device that searches for similar information, based on the provided information, the terminal device comprising: one or more memories; and circuitry which, in operation: obtains an instruction indicating which of an encrypted search, in which the server device performs the search using the information provided by the terminal device in an encrypted state, and an unencrypted search, in which the server device performs the search using the information provided by the terminal device in an unencrypted state, is to be performed; generates a first encrypted feature quantity by encrypting, using an encryption scheme dependent on the obtained instruction, a first feature quantity indicating a feature of obtained data, the encryption scheme dependent on the obtained instruction being a first encryption scheme that is homomorphic and makes decryption by the server device unsuccessful in a case where the instruction is an instruction to perform an encrypted search and being a second encryption scheme that makes decryption by the server device successful in a case where the instruction is an instruction to perform an unencrypted search; transmits the generated first encrypted feature quantity to the server device; generates an encrypted similarity value by encrypting, using the encryption scheme dependent on the instruction, a similarity value indicating a degree of similarity; transmits the generated encrypted similarity value to the server device; transmits the obtained instruction to the server device; receives encrypted reference data from the server device, the encrypted reference data being information obtained by encrypting reference data that is obtained as a result of the server device searching for similar information in accordance with the instruction; and decrypts the received encrypted reference data. 